For example, patent document 1 discloses a condition monitoring system (CMS) which monitors a condition of a mechanical element of a wind turbine. This condition monitoring system takes in a signal of a vibration sensor provided to the mechanical element, records over a long period of time how a quantity of a condition representing a state of vibration in a rated operation (hereafter referred to as a diagnosis parameter) varies with time, and based on the diagnosis parameter's increase rate, a feature of the variation and the like, determines whether the mechanical element has a failure (see patent document 1).